The present invention relates to a band-pass filter which can switch pass bands.
As a conventional band-pass filter of this type, a quartz filter or a piezoelectric ceramic filter has been used. In this band-pass filter, when pass bands are to be switched, a filter having a desired bandwidth is selected from a plurality of parallel-connected filters having different bandwidths by using a switching means such as a relay.
FIG. 3 shows an example of the band-pass filter described above. First and second filters 1 and 2 having different bandwidths are connected in parallel. In this arrangement, pass bands are switched from each other by selecting one of the filters 1 and 2 by using switches 3-1 and 3-2 synchronously operated in the same direction. In this case, the first filter 1 is constituted by six (n=6) quartz filter elements 1-1 to 1-6 which are connected as shown in FIG. 4. The second filter 2 is constituted by four (n=4) quartz filter elements similar in electrical connection to those shown in FIG. 4.
Such a band-pass filter, e.g., the band-pass filter shown in FIG. 3, requires an area and a volume corresponding to ten quartz filter elements because it is constituted by the first and second filters 1 and 2.
Recent mobile radio units (e.g., compact mobile radio units and portable telephone units), however, require smaller, lighter circuit arrangements. The use of the above-described band-pass filter, therefore, cannot satisfy such a requirement.